Duplex structure stainless steel having high strength and elongation and a process for producing the steel

ABSTRACT

A high strength and elongation stainless steel with a hardness of at least HV is formed of a duplex structure comprising 20% to 95% by volume of martensite having an average grain diameter of not more than 10 μm, with the balance being essentially ferrite, the steel including, by weight, up to 0.10% C, up to 2.0% Si, up to 4.0% Mn, up to 0.040% P, up to 0.010% S, up to 4.0% Ni, from 10.0% to 20.0% Cr, up to 0.12% N, more than 0.0050% to 0.0300% B, up to 0.02% O and up to 4.0% Cu, and optionally contains up to 0.20% A1, up to 3% Mo, up to 0.20% REM, up to 0.20% Y, up to 0.10% Ca, and up to 0.10% Mg, with the balance being Fe and unavoidable impurities.

FIELD OF THE INVENTION

The present invention relates to a high strength and elongationstainless steel having a dual phase structure consisting essentially offerrite and martensite that has good manufacturability and workability,and to a process for producing the steel, providing a high strengthstainless steel that is suitable for use as a material for forming intoshapes, such as by press-forming.

BACKGROUND OF THE FIELD

Chromium stainless steels containing chromium as a main alloying elementare classified into martensitic and ferritic stainless steels. Comparedwith austenitic stainless steel containing a relatively high amount ofnickel, they are inexpensive and feature such properties asferromagnetism and a low coefficient of thermal expansion. There aretherefore many applications in which chromium stainless steels are usednot only for economical reasons but also for their properties.

Particularly in the field of electronic instruments and precisionmachine parts where such chromium stainless steels are used, along withthe increasing demand of recent years, the requirements for steel sheetmaterials are becoming more rigorous. Steel sheet materials are requiredthat possess combinations of properties that may be in conflict, such asfor example high strength and high elongation, and good shape andthickness precision before working together with good shape precisionafter working.

Conventional chromium stainless steels having high strength includemartensitic stainless steels. For example, seven types of martensiticstainless steel are prescribed in the cold rolled stainless steel sheetsand strips of JIS G 4305. The prescribed carbon content of thesemartensitic stainless steels ranges from up to 0.08% (for SUS410S) to0.60-0.75% (for SUS440A), a high C content compared with ferriticstainless steels of the same Cr level. High strength can be imparted tothese steels by quenching treatment or by quenching and temperingtreatment. As indicated by the name, the structure of martensiticstainless sheets subjected to such heat treatment is basicallymartensitic. While this gives the steel great strength (hardness),elongation is extremely poor.

Accordingly, as martensitic steel that has been quenched (or quenchedand tempered) has poor workability, steel manufacturers usually ship thematerial in the annealed state, that is, as soft ferritic steel sheet orstrip having low strength and hardness, to a processor where thematerial is worked into product shape and is then subjected to quenchingor quenching and tempering treatment.

In many cases surface oxide film or scale formed by the post-formingheat treatment is undesirable with stainless steel in which the emphasisis on surface attractiveness. As a countermeasure, it therefore becomesnecessary to carry out the heat treatment in a vacuum or in an inert gasatmosphere, and to pickle and/or polish the steel after the heattreatment. Thus, using martensitic steel has tended to increase theburden on the side of the processor, unavoidably increasing the cost ofthe final product.

On the other hand, ferritic stainless steel has never been used much inapplications requiring high strength, and hardening by heat treatmenthas not been much expected. In some cases annealing is followed by workhardening using temper rolling (cold rolling) to obtain ferriticstainless steel having high strength. In this case the steel is used inthe cold rolled state, and a problem is that while increasing therolling reduction rate increases the strength, above a certain point theresult is a marked degradation in the elongation, meaning there is anupper limit to the level of strength at which a certain degree ofworkability can be maintained.

The properties of SUS430 strengthened by cold rolling at 20-30%, forexample, show a poor strength-elongation balance, with a hardness ofaround HV 230 and no more than 2 or 3% elongation. Moreover, usingtemper rolling to obtain wide material formed to a good shape is itselfdifficult, and the material exhibits considerable plane anisotropyregarding strength and elongation, making it difficult to obtain goodshape precision after working.

To solve the above problems of conventional high strength chromiumstainless steels, the present inventors have proposed, for example inJP-A-63-7338, JP-A-63-169330 to JP-A-63-169335, JP-A-1-172524 andJP-A-1-172525, a process for the production of a strip of a chromiumstainless steel of a duplex structure consisting essentially of ferriteand martensite and having high strength and elongation, which processcomprises the steps of basically hot rolling and cold rolling a slab ofa steel to provide steel strip, said steel having a composition adjustedto form a structure of ferrite and austenite at high temperature,continuous finish heat treatment in which the steel strip is heated toan appropriate temperature above the Ac₁ point of the steel to form atwo-phase of ferrite and austenite and maintained at that temperature,and the heated strip is cooled at an appropriate cooling rate totransform the austenite to martensite.

OBJECT OF THE INVENTION

The duplex structure chromium stainless steel strip according to thisinvention has fully sufficient properties for use as a high strengthmaterial for forming into shapes, i.e., a good balance between strengthand elongation, low plane anisotropy with respect to strength andelongation and a low yield strength and yield ratio, thus solving allthe problems of conventional high strength chromium stainless steels.

However, in the manufacturing process there are cases in which suchduplex structure stainless steel strip exhibit a hot workability that isinferior to that of conventional ferritic and martensitic stainlesssteels. This is because the duplex structure stainless steels are hotrolled in a state of coexistence of ferrite and austenite, which exhibitbasically different deformabilities and deformation resistances duringhot rolling, and the hot workability is affected by the ratio andhigh-temperature strength of the two phases. Taking for example theratio of the two phases, at high temperatures duplex structure stainlesssteels have less ferrite than conventional ferritic stainless steels,which tends to degrade the hot workability. On the other hand, withstainless steels with a completely martensitic structure that formssingle-phase austenite during hot rolling, this degradation of hotworkability owing to the coexistence of the two phases does notconstitute a problem.

Degradation of the hot workability can give rise to fine cracking atedge portions of the steel strip during hot rolling. Fine cracking atedge portions (hereinafter also referred to simply as "edge cracking")of hot rolled steel strip occurs particularly when the proportion ofmartensite is increased for higher strength, that is, when a compositionbalance is used that increases the amount of austenite formed at hightemperatures.

Although edge cracking does not adversely affect the properties of thematerial, it can cause breakage of the steel strip during the coldrolling step that follows. It is therefore necessary to remove edgecracking prior to the cold rolling, which tends to reduce the widthyield. To prevent this happening the number of hot rolling passes can beraised, as required, reducing the rolling rate of reduction per pass.However, this is all a hindrance to the economic aspects that are afeature of duplex structure stainless steel strips. The object of thepresent invention is to solve such problems.

DISCLOSURE OF THE INVENTION

In accordance with this invention, there is provided high strength andelongation stainless steel having a duplex structure of from 20% to 95%by volume of martensite with an average grain diameter of not more than10 μm, with the balance being essentially ferrite, and having a hardnessof at least HV 200, said steel comprising, by weight:

up to 0.10% C,

up to 2.0% Si,

up to 4.0% Mn,

up to 0.040% P,

up to 0.010% S,

up to 4.0% Ni,

from 10.0% to 20.0% Cr,

up to 0.12% N,

more than 0.0050% to 0.0300% B,

up to 0.02% O, and

up to 4.0% Cu,

and optionally containing one or more selected from up to 0.20% A1, upto 3% Mo, up to 0.20% REM, up to 0.20% Y, up to 0.10% Ca, and up to0.10% Mg, to satisfy

0.01%≦C +N≦0.20%

0.20%≦Ni+(Mn+Cu)/3≦5.0%,

the balance being Fe and unavoidable impurities.

In accordance with this invention, cold rolled steel strip is producedfrom the above composition-controlled steel slab by a hot rolling stepcomprising rough rolling and finish rolling, and a cold rolling step.The cold rolled strip is then subjected to dual-phase heat treatmentcomprising passing the strip through a continuous heat treatment furnacewhere it is heated to a temperature ranging from at least 100° C. abovethe Ac₁ point of the steel to 1100° C. to form a two-phase of ferriteand austenite and maintained at that temperature for not longer than 10minutes, and cooling it from the maximum heating temperature to ambienttemperature at an average cooling rate of from at least 1° C./s to notmore than 1000° C./s, thereby producing stainless steel strip having theabove duplex structure and hardness. With respect to the content amountsof C, N, Ni, Mn, Cu, Cr, and Si in the steel of the composition of thisinvention, in accordance with the equation ##EQU1## the values of γmaxcan be divided into case A) when content values are used to satisfy arelationship of up to 65, and case B) when content values are used tosatisfy a relationship of more than 65 to not more than 95. For theformer, case A), the martensite content in the duplex structure is from20% to not more than 70% by volume and the hardness is at least HV 200.For the latter, case B), the martensite content in the duplex structureis from 60% to not more than 95% by volume and the hardness is at leastHV 320.

In the hot rolling process, material according to case A can be givenfour or more rough rolling passes at a reduction rate of at least 30%per pass, while material according to case B can be given three or morerough rolling passes at a reduction rate of at least 30%.

DETAILED DESCRIPTION OF THE INVENTION

Based on extensive research into the production of steel having a duplexstructure of ferrite and martensite in the strip form, to find whatcompositional balance would provide such a duplex structure withoutgiving rise to edge cracking, the present inventors discovered thecomposition and manufacturing conditions that enable this object to beattained.

The reasons for the limitations on the chemical components of the steelspecified by the invention, and the steps of the manufacturing process,will now be described in specific detail together with the functionthereof.

C and N are strong and inexpensive austenite formers when compared withNi, Mn, Cu and the like, and have an ability to greatly strengthenmartensite. Accordingly, they are effective to control and increase thestrength of the product subjected to heat treatment in a continuous heattreatment furnace to obtain a duplex structure. Thus, to obtain a duplexstructure from the continuous heat treatment step that consistsessentially of ferrite and martensite having the required high strengthand good elongation, it is necessary to add at least 0.01% (C+N) evenwhen austenite formers such as Ni, Mn and Cu are added. However, anexcessively high (C+N) content will increase the amount of martensiteformed by the heat treatment, perhaps even to the extent that thestructure becomes 100% martensitic, and the hardness of the martensitephase itself becomes unduly high, so that while high strength may beattained, elongation is degraded. It is therefore necessary for the(C+N) content to be up to 0.20%, and to satisfy the condition0.01%≦(C+N)≦0.20%.

A high C content tends to reduce toughness and have an adverse effect onmanufacturability and product properties. Also, in the dual-phase heattreatment in which, using a continuous heating furnace, the steel isheated to a temperature at which a two-phase structure of ferrite andaustenite is formed and is then quenched, during the cooling step Crcarbides dissolved during the heating reprecipitate at ferrite andaustenite (martensite;, after cooling) grain boundaries, so-calledsensitization, and the resultant layer of chromium depletion in areasimmediately adjacent to grain boundaries markedly reduces corrosionresistance. Hence, a C content of up to 0.10% has been specified.

Solubility factors makes it difficult to add a high amount of N, andhigh added N can cause an increase in surface defects. Thus, the upperlimit for N has been set at 0.12%.

Si is a ferrite former and also acts as a powerful solid solutionstrengthener in both the ferritic and the martensitic phases. As such,Si is effective for controlling the amount of martensite, and the degreeof strength. The upper limit for Si is set at 2.0%, since adding a largeamount of Si adversely affects hot and cold workability.

Mn, Ni and Cu are austenite formers and are effective for controllingthe strength of the steel and the amount of martensite after dual-phaseheat treatment. Moreover, adding Ni, Mn or Cu makes it possible toreduce the C content. By producing a softer martensite, this improvesthe elongation and, by suppressing precipitation of Cr carbides at grainboundaries, also makes it possible to prevent degradation of corrosionresistance caused by sensitization.

Ni, Mn and Cu also have the effect of markedly lowering the Ac₁ point ofthe steel, that is, the temperature at which the austenitic phase startsto form during heating. This has a major significance in terms ofimproving the workability of the fine mixed structure (of ferrite andmartensite) that is a feature of this invention.

In the duplex structure stainless steel at which this invention isdirected, the duplex structure is obtained by the production of anaustenite phase in a ferrite matrix during dual-phase heat treatmentthat follows the cold rolling. In order to obtain a fine structure, itis necessary to finely distribute the austenite phase that is formed. Todo this, (1) it is important to effect dual-phase heat treatment of thesteel in the as-cold-rolled state by rapid heating in a continuousheating furnace and form the austenite at the same time in the ferritematrix (i.e., to increase the austenite nuclei formation sites) in whichthere is residual strain from the cold rolling. An effective way ofaccomplishing this more actively is (2) to use constituents having anAc₁ point that is close to, or not higher than, the ferrite phaserecrystallization temperature. For this, it is both necessary andeffective to add Ni, Mn or Cu, as these elements lower the Ac₁ point.

Even when dual-phase heat treatment of the steel in the as-cold-rolledstate is applied, in cases where the Ac₁ point is quite higher than theferrite phase recrystallization temperature, the onset of austeniteformation takes place after full recrystallization of the ferrite phase.In such a case, austenite nuclei formation sites are limited to theferrite grain boundaries, resulting in enlargement of the martensite.

Ni has the greatest effect on austenite forming ability per unit masspercent and on the Ac₁ point; Mn or Cu has only about one-third theeffect that Ni has. Therefore, the formula Ni+(Mn+Cu)/3 is used todetermine the amount of Ni, Mn and Cu to add to obtain the above effect,for which said added amount needs to be at least 0.2%. On the otherhand, adding a large amount of Ni would make the product uneconomicallycostly. Therefore, the content of each of Ni, Mn and Cu on an individualbasis is set at up to 4.0%, and at up to 5.0% in the case ofNi+(Mn+Cu)/3.

P is an element that has a powerful solid solution strengthening effect,but as it can also have an adverse effect on toughness, it is limited tono more than 0.040%, the amount permitted in normal practice.

The lower the S content the better, since this is an element that isundesirable with respect to edge cracking and corrosion resistance. Witha S content of less than 0.0010%, there is no edge cracking, evenwithout the addition of B, described below. However, since in the caseof a commercial-scale steel manufacturing reducing S to a stablyultralow level would actually have the effect of increasing themanufacturing cost, an upper limit of 0.010% S is permitted.

Cr is the most important element with respect to the corrosionresistance of stainless steel, and must be contained in an amount of atleast 10.0% to achieve the desired level of corrosion resistance for astainless steel. However, too high a Cr content increases the amounts ofaustenite formers required to form the martensite phase and achieve highstrength, raises the product cost, and reduces toughness andworkability. Accordingly, the upper limit for Cr is set at 20.0%.

The addition of B is an important part of this invention, because it ishighly effective for preventing edge cracking in the hot rolled steelstrip of this invention. This effect also makes it possible to increasethe reduction rate per hot rolling pass, which improves productionefficiency by reducing the number of rough rolling passes.

Edge cracking in the duplex structure stainless steel strip of thisinvention is caused by differences between the deformability anddeformation resistance (high-temperature strength) of the ferrite andaustenite phases at the hot-rolling temperature region. Cracking occursat the interface between the phases during hot rolling when, as a resultof the differences, the burden on the interface between the phasesbecomes too large for the interface to match the deformation. Anothercontributory factor is embrittlement occurring at the phase interfaceresulting from the quantitative ratios of the two phases and Ssegregation at the interface boundary. B has the effect of inhibitingthis. Although it is not yet clear why B has this effect, it might bethat as boron itself has a tendency toward boundary segregation, theaddition of boron reduces S segregation, or it might be that the boronitself increases the strength of the interface. A boron content of0.0050% or less may not effectively prevent edge cracking, while moretitan 0.0300% may cause deterioration of surface properties. Thus, aboron content of more than 0.0050% to not more than 0.0300% isspecified.

O forms oxide non-metallic inclusions, which impairs the purity of thesteel, and has an adverse affect on bendability and press formability,so the 0 content has been set at not more than 0.02%.

A1 is effective for deoxygenation during the steel-making process, andserves to remarkably reduce A₂ inclusions which adversely affect thepress formability of the steel. However, an A1 content that exceeds0.20% has a saturation effect and tends to increase surface defects, so0.20% has been set as the upper limit for A1.

Mo is effective for enhancing the corrosion resistance of the steel.However, a high Mo content degrades hot workability and increasesproduct cost, so the upper limit for Mo has been set at 3.0%.

REM (rare earth metals), Y, Ca and Mg are effective elements forimproving hot workability and oxidation resistance. However, in eachcase the effect is saturated if too much is added. Accordingly, an upperlimit of 0.20% has been set for REM and for Y, and an upper limit of0.10% has been set for Ca and for Mg.

The γmax value calculated according to equation (1) is an indexcorresponding to the maximum amount, in percent, of austenite at hightemperature. It therefore follows that γmax controls the amount ofmartensite formed after the dual-phase heat treatment and affects thehot workability. With a γmax that does not exceed 65, edge cracking doesnot constitute much of a problem, while improved hot workabilityresulting from reduced S and the addition of B makes it possible toperform the hot rough rolling using four or more passes at a reductionrate of at least 30% per pass, thereby enabling the number of hotrolling passes to be reduced.

With a γmax that exceeds 65, hot workability is reduced, but owing tothe decrease in S and the addition of B and also of REM, Y, Ca and Mg,hot rough rolling can be performed in three passes at a reduction rateof at least 30% per pass without giving rise to edge cracking. If γmaxis too high, the amount of martensite following the dual-phase heattreatment will be close to 100%, with a departure from the object of theduplex structure stainless steel, that of attaining both high strengthand high elongation. Therefore, an upper limit of 95 has been set forγmax.

The amount of martensite following the dual-phase heat treatment is themain factor determining the strength (hardness) of the steel. While anincrease in the amount of martensite increases the strength of thesteel, the elongation decreases. The maximum amount of martensite thatis produced can be controlled, for example, by the compositional balancerepresented by γmax. Even using identical compositions, the amount ofmartensite can be varied by the dual-phase heat treatment, in particularby the heating temperature used. If the amount of martensite is lessthan 20% by volume, it is difficult to attain a hardness of at least HV200, while on the other hand, more than 95% by volume of martensiteresults in a major decrease in ductility, hence a low absoluteelongation. In each case the significance of the two-phase structure offerrite and martensite is lost. Thus, the amount of martensite followingthe dual-phase heat treatment has been set at from not less than 20% tonot more than 95% by volume.

The metallographic fineness of the duplex structure steel of thisinvention has a bearing on the degree of workability. Specifically, afiner structure results in enhanced bending workability. It is possiblethat this is because with finer grains, local concentrations ofprocessing stresses are alleviated and uniformly dispersed. While it isdifficult to definitively define the metallographic size of duplexstructure steel, an average martensite grain diameter of not more than10 μm markedly improves the bending workability, as shown in theexamples described below. Thus, not more than 10 μm has been set as anindex for the average grain size of the martensite phase.

The manufacturing conditions for the duplex structure steel stripaccording to this invention will now be described.

A slab of a stainless steel of the above-described adjusted chemicalcomposition is prepared using conventional steel-making and castingconditions, and is subjected to hot rolling comprising rough rolling andfinish rolling, to provide a hot rolled strip. A steel having thecomposition range prescribed by this invention, with a good rollabilityγmax of not more than 65, can be subjected to four or more rough rollingpasses at an average reduction rate of at least 30% per pass, while asteel with a γmax of from more than 65 to not more than 95 can besubjected to three or more rough rolling passes at an average reductionrate of 30% per pass, thereby enhancing production efficiency andproviding hot rolled strip with no edge cracking.

The hot rolled strip is preferably annealed and descaled. Although theannealing is not essential, it is desirable as it not only softens thematerial to enhance the cold rollability of the hot rolled strip, butalso transforms and decomposes intermediately transformed phase(portions which were austenite at the high temperatures) in the hotrolled strip to ferrite and carbides, thereby producing strip that,after cold rolling and dual-phase heat treatment, has a uniform duplexstructure. Descaling can be done by a conventional pickling process.

The hot rolled strip is then cold rolled to a product thickness. Thecold rolling step may be carried out as a single cold rolling with nointermediate annealing, or as two cold rollings separated by anintermediate annealing. An intermediate annealing increases the cost andis not an essential requirement. However, intermediate annealing isadvantageous in that it reduces the plane anisotropy of the product. Itis preferable to use an intermediate annealing temperature (materialtemperature) that is not higher than the Ac₁ point of a single phaseferrite formation zone where there is no austenite. If the annealingshould however be done above the Ac₁ point at which ferrite andaustenite are formed, it is desirable to use a temperature zone notabove about 850° C. where the proportion of austenite is low.

Dual-phase heat treatment comprises passing the cold rolled stripthrough a continuous heat treatment furnace to obtain the aforementionedfine structure. Heating the steel at a temperature zone at which atwo-phase of ferrite and austenite is formed is an essential conditionfor obtaining heat treated steel having a mixed structure of ferrite andmartensite. In the process of this invention, when the continuous heattreatment furnace is being heated up, at a temperature near the Ac₁point at which the austenite starts to form, changes in temperature canresult in large variations in the amount of austenite formed, which isto say, in the amount of martensite formed by the subsequent cooling, sothat in some cases a desired hardness (strength) is not stably obtained.

In the composition range of the steel of this invention, substantiallysuch variations in hardness do not arise if a heating temperature of atleast about 100° C. above the Ac₁ point of the steel is used. Thus, apreferable heating temperature in the dual-phase heat treatment of theinvention is at least about 100° C. above the Ac₁ point of the steel. Ifthe heating temperature is too high, the hardening effect becomessaturated and may even be decreased, and it is also disadvantageous interms of cost. Accordingly, the upper limit for the heating temperaturehas been set at 1100° C.

At a temperature at which a two-phase structure of ferrite and austeniteis formed during the dual-phase heat treatment, the austenite formationamount reaches equilibrium within a shoal period of time. Thus, theheating time can be as short as not more than about 10 minutes. Thecooling rate in the dual-phase heat treatment should be sufficient totransform the austenite to martensite. For this, a cooling rate of atleast 1° C./s is required. A cooling rate above about 1000° C./s is notpractical, so a cooling rate of from 1° C./s to 1000° C./s isprescribed. The cooling rate is expressed as an average cooling ratefrom the maximum heating temperature to the ambient temperature. Oncethe transformation from austenite to martensite has taken place, it isno longer necessary to employ the said cooling rate.

EXAMPLES

Steels having the chemical compositions shown in Table 1 were vacuummelted to form 400 kg slabs 165 mm thick, 200 mm wide. The slabs weredivided into two as required, heated to 1200° C., rough rolled, usingthe number of passes shown in Table 2, and finish rolled at 920° C. to afinish sheet thickness of 3.6 mm. After the hot rolling the sheets wereexamined for edge cracking. The results are shown in Table 2

                                      TABLE 1                                     __________________________________________________________________________    Steel                                                                         No. C  Si Mn P  S    Ni Cr N  B    O    Al Cu Other γmax                                                                        Category              __________________________________________________________________________    1   0.035                                                                            0.40                                                                             0.22                                                                             0.028                                                                            0.0032                                                                             0.55                                                                             15.85                                                                            0.012                                                                            0.0073                                                                             0.0069                                                                             0.005                                                                            0.42     40.4                                                                              Inventive             2   0.029                                                                            0.56                                                                             0.26                                                                             0.027                                                                            0.0010                                                                             1.54                                                                             16.28                                                                            0.011                                                                            0.0061                                                                             0.0032                                                                             0.021                                                                            0.04     50.3                                                                              "                     3   0.077                                                                            1.45                                                                             0.82                                                                             0.017                                                                            0.0028                                                                             0.34                                                                             17.06                                                                            0.025                                                                            0.0065                                                                             0.0043                                                                             0.018                                                                            2.25     54.0                                                                              "                     4   0.063                                                                            0.50                                                                             2.30                                                                             0.029                                                                            0.0019                                                                             1.03                                                                             16.15                                                                            0.063                                                                            0.0220                                                                             0.0035                                                                             0.022                                                                            0.12                                                                             Mo: 2.06                                                                            69.7                                                                              "                     5   0.065                                                                            0.53                                                                             0.31                                                                             0.024                                                                            0.0015                                                                             2.01                                                                             16.21                                                                            0.009                                                                            0.0094                                                                             0.0061                                                                             0.011                                                                            0.03                                                                             Ca: 0.008                                                                           80.0                                                                              "                                                                   Mg: 0.01                        6   0.042                                                                            0.41                                                                             0.83                                                                             0.025                                                                            0.0012                                                                             0.31                                                                             11.97                                                                            0.012                                                                            0.0085                                                                             0.0064                                                                             0.007                                                                            0.05     83.3                                                                              "                     7   0.076                                                                            0.52                                                                             0.27                                                                             0.026                                                                            0.0005                                                                             2.58                                                                             16.40                                                                            0.010                                                                            0.0071                                                                             0.0023                                                                             0.042                                                                            0.05                                                                             REM: 0.04                                                                           92.7                                                                              "                                                                   Y: 0.07                         8   0.089                                                                            0.52                                                                             0.20                                                                             0.023                                                                            0.0027                                                                             0.05                                                                             16.30                                                                            0.054                                                                            0.0067                                                                             0.0054                                                                             0.009                                                                            0.03     61.2                                                                              Comparative           9   0.032                                                                            0.52                                                                             0.30                                                                             0.027                                                                            0.0016                                                                             1.48                                                                             16.31                                                                            0.008                                                                             0.0002*                                                                           0.0065                                                                             0.007                                                                            0.05     49.2                                                                              "                     10  0.080                                                                            0.54                                                                             0.28                                                                             0.022                                                                            0.0024                                                                             2.03                                                                             16.32                                                                            0.011                                                                            0.0014                                                                             0.0072                                                                             0.005                                                                            0.05     81.2                                                                              "                     11  0.091                                                                            0.55                                                                             0.31                                                                             0.027                                                                            0.0047                                                                             2.25                                                                             16.42                                                                            0.013                                                                             0.0003*                                                                           0.0033                                                                             0.010                                                                            0.04     92.5                                                                              "                     __________________________________________________________________________     *Not added                                                               

                  TABLE 2                                                         ______________________________________                                                                    Rough                                                       Example   Steel   rolling Edge                                      Category  No.       No.     passes*.sup.1                                                                         Cracking*.sup.2                           ______________________________________                                        Inventive 1         1       (5/5)   ◯                                       2         2       (3/7)   ◯                                       3         2       (5/5)   ◯                                       4         3       (5/5)   ◯                                       5         4       (3/7)   ◯                                       7         5       (3/7)   ◯                                       8         6       (3/7)   ◯                                       9         7       (3/7)   ◯                             Comparative                                                                             1         8       (3/7)   ◯                                       2         9       (3/7)   ◯                                       3         9       (5/5)   X                                                   4         10      (3/7)   X                                                   5         11      (3/7)   X                                         ______________________________________                                         *.sup.1 (No. of passes at reduction of 30% or more/Total no. of passes)       *.sup.2 ◯: No edge cracking                                        .sup. X: Edge cracking                                                  

As can be seen from the results in Table 2, inventive steels Nos. 1 to 7could be hot rolled without edge cracking occurring, even in the casesof steels Nos. 1 to 3, formed using a γmax value not exceeding 65 andrough rolled at high reduction rates.

In contrast, high-reduction rough rolling of comparative steel No. 9resulted in edge cracking, showing it to have inferior hot workabilitycompared to inventive steel No. 2 which has substantially the same γmax.Comparative steels Nos. 10 and 11 also showed edge cracking, owing totheir low boron content, showing them to have inferior hot workabilitycompared to inventive steels Nos. 5 and 7, respectively, with virtuallythe same γmax.

Hot rolled strips of inventive steels Nos. 1 to 7 that exhibited no hotworkability problems, and inventive steel No. 8, were then annealed byheating at 780° C. for 6 hours and cooling in-furnace, pickled, and coldrolled to form strip 0.7 mm thick. The cold rolled strips were thensubjected to dual-phase heat treatment in a continuous heat treatmentfurnace, using the conditions shown in Table 3, which also shows thematerial properties thus obtained.

                                      TABLE 3                                     __________________________________________________________________________                    Dual-phase Heat Treatment                                                                     Steel Properties                                                         Cooling                                                                            Amount of                                                                           Martensite                                     Example                                                                            Steel                                                                             Temperature                                                                          Time                                                                              rate martensite                                                                          grain size                                                                          Hardness                                                                            Elongation                         No.  No. (°C.)                                                                         (min.)                                                                            (°C./s)                                                                     (%)   (μm)                                                                             (HV)  (%)   Bendability*.sup.1    __________________________________________________________________________    Inventive                                                                            10   1   1000   3   50   38    8     231   29    ◯                11   2    980   3   50   52    7     276   16    ◯                12   3    950   3   20   50    5     323   14    ◯                13   4   1000   5   20   64    5     335   14    ◯                14   5   1050   5   10   82    5     388   12    ◯                15   6    950   3   20   75    5     352   11    ◯                16   7   1000   3   20   89    5     426   10    ◯         Comparative                                                                           6   8   1000   3   20   58    14    297   15    X                             7   6    .sup. 800*.sup.2                                                                    3   20    0    --    151   28    ◯                 8   6    .sup. 1000*.sup.3                                                                   3   20   72    20    343   12    X                     __________________________________________________________________________     *.sup.1 Ability to be bent over flat along with rolling direction along       the ridge line                                                                 .sup. ◯: No problem                                               .sup. X: Cracks                                                              *.sup.2 Annealing at ferrite zone                                             *.sup.3 Strip of Example No. 7, subjected to dualphase heat treatment         after annealing                                                          

As can be seen from Table 3, in accordance with the process of thisinvention, high strength duplex structure steel strips were obtainedthat exhibited good workability and elongation, having an averagemartensite grain diameter of not more than 10 μm. In contrast, owing tolow Ni, Mn and Cu contents, the Ni+(Mn+Cu)/3 value of comparativeexample No. 6 (steel No. 8) was 0.13, lower than the range according tothe present invention, resulting in a relatively large formed martensitegrain size of 14 μm, hence poor bendability. Comparative example No. 7(steel No. 6) was subjected to dual-phase heat treatment at a lowtemperature of 800° C., whereby annealing took place at a ferrite regionand martensite formation did not take place, resulting in low strength.Comparative example No. 8 was given this annealing and then subjected tofurther dual-phase heat treatment at 1000° C. However, this releasesprocessing stresses imposed by the cold rolling, causing austenite toform at recrystallized ferrite grain boundaries. This coarsens themartensite formed by the cooling, degrading the bendability.

Thus, as described in the foregoing, in accordance with the process ofthis invention, high strength stainless steel sheet materials having ahardness of at least HV 200 and exhibiting good elongation as well asgood workability, can be commercially and economically produced in theform of steel strips, and as such can be widely applied in fields suchas electronic instruments and precision machine parts in which highstrength and workability are required.

What is claimed is:
 1. A stainless steel of a duplex structure,comprised of from 60% to 95% by volume of martensite having an averagegrain diameter of not more than 10 μm, with the a balance beingessentially ferrite, said steel having a high strength and an elongationas well as a hardness of at least HV 320, said steel comprising, byweight:up to 0.10% C, up to 2.0% Si, up to 4.0% Mn, up to 0.040% P, upto 0.010% S, up to 4.0%Ni, from 10.0% to 20.0% Cr, up to 0.12% N; morethan 0.0050% to 0.0300%B, up to 0.02% O, and up to 4.0% Cu, the balancebeing Fe and unavoidable impurities, and satisfying 0.01%≦C+N≦0.20%0.20%≦Ni+(Mn+Cu)/3≦5.0% and the content amounts of C, N, Ni, Mn, Cu, Crand Si in the steel satisfy a relationship for a γmax value of more than65 and not more than 95, obtained from the equation

    γmax=420 (%C)+470 (%N)+23 (%Ni) +7 (%Mn)+9 (%C)-11.5 (%Cr) -11.5 (%Si)+189.


2. 2. The stainless steel of claim 1 wherein the steel further comprisesone or more selected from up to 0.20% A1, up to 3% Mo, up to 0.20% REM,up to 0.20% Y, up to 0.10% Ca and up to 0.10% Mg.
 3. A process for theproduction of a stainless steel of a duplex structure, comprised of from60% to 95% by volume of martensite having an average diameter not morethan 10 μm, with the balance being essentially ferrite, said steelhaving high strength and elongation as well as a hardness of at least HV320, which process comprises;a step of hot rolling a slab of a steel byrough rolling and finish rolling to provide a hit rolled strip, saidsteel comprising, by weight, up to 0.10% C, up to 2.0% Si, up to 4.0%Mn, up to 0.040% P, up to 0.010% S, up to 4.0% Ni, from 10.0% to 20.0%Cr, up to 0.12% N, more than 0.0050% to 0.0300%B, up to 0.02% O, and upto 4.0% Cu, the balance being Fe and unavoidable impurities, andsatisfying 0.01%≦C+N≦0.20% 0.20%≦Ni+(Mn+Cu)/3≦5.0% and the contentamounts of C, N, Ni, Mn, Cu, Cr and Si in the steel satisfy arelationship for a γmax value of more than 65 and not more than 95,obtained from the equation ##EQU2## a step of cold rolling the hotrolled strip to provide a cold rolled strip, and a step of dual-phaseheat treatment in which the cold rolled strip is passed through aheating zone where it is heated to a temperature ranging from at leastabout 100° C. above the Ac1 point of the steel to 1100° C. to form atwo-phase of ferrite and austenite and maintained at that temperaturefor not longer than 10 minutes, and the heated strip is cooled from themaximum heating temperature to ambient temperature at an average coolingrate of at least 1° C./s to not more than 1000° C./s.
 4. The process inaccordance with claim 3 wherein the rough rolling of the hot rollingstep is carried out in three or more passes at a reduction rate of notless than 30% per pass, the duplex structure is comprised of from 60% to95% by volume of martensite having an average grain diameter of not morethan 10 μm, with the balance being essentially ferrite, the hardness isat least HV 320 and the content amounts of C, N, Ni, Mn, Cu, Cr, and Siin the steel satisfy a relationship for a γmax value of more than 65 tonot more than 95, obtained from the equation ##EQU3##
 5. A process forthe production of a stainless steel of a duplex structure, comprised offrom 60% to 95% by volume of martensite having an average grain diameterof not more than 10 μm, with the balance being essentially ferrite,having high strength and elongation as well as a hardness of at least HV320, which process comprises:a step of hot rolling a slab of a steel byrough rolling and finish rolling to provide a hot rolled strip, saidsteel comprising, by weight, up to 0.10% C, up to 2.0% Si, up to 4.0%Mn, up to 0.040% P, up to 0.010% S, up to 4.0% Ni, from 10.0% to 20.0%Cr, up to 0.12% N, more than 0.0050% to 0.0300% B, up to 0.02% O, and upto 4.0% Cu, the balance being Fe and unavoidable impurities, andsatisfying
 0. 01%≦C+N≦0.20%0.20%≦Ni+(Mn+Cu)/3≦5.0%. a step of coldrolling the hot rolled strip to provide a cold rolled strip, and a stepof dual-phase heat treatment in which the cold rolled strip is passedthrough a heating zone where it is heated to a temperature ranging fromat least 100° C. above the Ac₁ point of the steel to 1100° C. to form atwo-phase of ferrite and austenite and maintained at that temperaturefor not longer than 10 minutes, and the heated strip is cooled from themaximum heating temperature to ambient temperature at an average coolingrate of at least 1 ° C./s to not more than 1000° C./s.
 6. The process inaccordance with claim 5 wherein the rough rolling of the hot rollingstep is carried out in three or more passes at a reduction rate of notless than 30% per pass, and the content amounts of C, N, Ni, Mn, Cu, Cr,and Si in the steel satisfy a relationship for a γmax value of more than65 to not more than 95, obtained from the equation ##EQU4##
 7. Theprocess of claim 5 wherein the boron lower limit is 0.0061%.
 8. Thestainless steel of claim 1 wherein the boron lower limit is 0.0085%. 9.The process of claim 3 wherein the boron lower limit is 0.0085%.